Optical signals suffer degradation between the transmitter and receiver from such factors as noise, inter-symbol interference, fiber dispersion, non-linearity of the elements and transmission medium. In addition, in amplified wavelength division multiplexed (WDM) systems, the transmission characteristics vary from one channel to another due to the non-flat gain and noise profile of erbium-doped fiber amplifiers (EDFAs).
Distortion is defined as any inaccurate replication of a signal transmitted over a communication link, and could originate in any network element (NE) along the link. It can be measured by assessing the difference between the wave shape of the original signal and that of the signal at the network element of interest, after it has traversed the transmission link.
In the last decade, transmission rates of data signals have increased progressively, which has led to more complex and less tolerant transmission systems. For transmission at high rates, such as 40 or 80 Gb/s, the distortion of the optical link is a critical parameter. With various types of dispersion shifted fiber, dispersion compensating fiber and dispersion compensating elements that make up a given link, determining the cause of a distortion in the received signal is no longer a simple operation, especially in optical transmission systems with in-line optical amplifiers. System performance degradation caused by noise and optical path distortions are also usually difficult to separate, making the performance evaluation complicated.
In the evaluation of the characteristics of an optical fiber communication system, the optical signal to noise ratio (OSNR) has been used as a parameter for performance evaluation. This parameter is particularly used in networks which utilise optical (photonic) switching arrangements for the routing or forwarding of the user data, as the signals then remain in the optical domain as they traverse the network. The signal to noise ratio is typically determined by analysing the optical spectrum at the location of interest.
The optical switching arrangement in the node of such a network comprises a photonic cross connect, which is an optical switching fabric for selectively routing signals at the inputs to the outputs. The inputs may be provided with individual channels or with grouped bands of channels. In either case, a multiplexing/demultiplexing arrangement is provided between the fiber whiz carries the WDM signal and the cross connect ports.
In such networks, optical noise from spans leading up to a node passes through the node after being shaped by the multiplexers/demultiplexers. Once a noise component has passed through a node, it no longer has a constant slope frequency spectrum. Instead, the noise component follows the shape of the multiplexer/demultiplexer filtering function. However, conventional optical signal to noise ratio measurements rely on the noise having constant slope. One known technique involves measuring the signal level at frequencies on either side of the channel of interest. This signal is considered to comprise only noise—since no signal is intended to be present at these inter-channel frequencies. A constant slope noise floor is assumed across frequency, to interpolate the noise appearing at the channel frequency. This gives an inaccurate indication of the noise level when there has been noise shaping as described above.
Accurate knowledge of the optical signal to noise ratio is, however, required to enable accurate fault finding and analysis.
It has also been proposed to measure OSNR using polarisation extinction techniques. These techniques rely upon the fact that a data signal has a definite polarisation state, whereas noise is distributed over all polarisation states. Signal strength and noise level is thus measured using a polariser. However, it is difficult to achieve sufficient extinction on of the data signal when measuring the much smaller noise level, and the polarisation state of a signal will evolve over time. These aspects make the process complicated and inaccurate.
Time domain distinction techniques have also been proposed, by which the data signal is gated on and off. If the signal is gated sufficiency rapidly (faster than the response time of the Erbium doped fibre amplifiers within the system), the noise will still be present when the signal is turned off, and can therefore be measured in isolation. This enables the noise to be measured at the channel frequency, but requires interruption of the data signal and can not therefore be used in live systems.
There is a need for an accurate OSNR measurement system which can be used during signal transmission, which takes account of noise shaping resulting from optical filtering.
In addition to OSNR, the performance of an optical system is also often defined by a parameter called Q. The Q value (or Q-factor) indicates the ‘useful signal’-to-noise ratio of the electric signal regenerated by the optical receiver, and is defined as follows:   Q  =                    μ        1            -              μ        0                            σ        1            +              σ        2            
where μ1 is the mean value of the ‘1’s, μ0 is the mean value of the ‘0’s, σ1 is the standard deviation of the level of ‘1’s, and σ0 is the standard deviation of the level of ‘0’s. These parameters can be understood from looking at the so-called eye diagram, which represents the received signal, time-shifted by integer multiples of the bit period, and overlaid. The eye diagram can be produced on an oscilloscope by applying a baseband signal to the vertical input of the oscilloscope and triggering the instrument time base at the symbol rate. For a binary signal, such an eye diagram has a single ‘eye’, which is open or closed to an extent determined by the signal degradation. An open pattern is desired, as this provides the greatest distance between signals representing a 1 and those representing a 0. Changes in the eye size indicate inter-symbol interference, amplitude irregularities, or timing problem, such as jitter, depending on the signal that is measured. The value of Q can be used directly to derive the bit error ratio, and various techniques are available for monitoring the Q value.
These techniques require conversion of the signal from the optical to the electrical domain. One preferred technique involves sweeping the decision threshold of the receiver through all voltages from the voltage level corresponding to a zero to the voltage level corresponding to a one. For example, when the decision threshold is near the zero voltage level, there will be no errors in interpreting a “1”, even if there is significant distortion. There will, however be a greatly increased error ratio in interpreting the zeros. The Bit Error Ratio (BER) is measured for each decision threshold voltage, and by mapping the BER values using an appropriate function, a straight line extrapolation can be used to obtain the Q value.
This Q value provides au extremely useful measurement tool, for example for locating errors in the network, which manifest themselves as a step change in Q value, but which may not be detectable by other techniques, for example errors resulting from channel crosstalk. A problem arises that Q cannot be measured without opto-electric conversion circuitry, and cannot be measured at amplifier sites without providing additional circuitry.